This phrase appears to mean “told the truth”, “was frank”.
It seems to have been around for quite a while but I only just encountered it.
Some samplings from the wild:
“ Willow Smith kept it a buck about how she really feels about monogamy”
“The fact that you kept it a buck about using his beat gets my respect.”
“The CEO of TDE took to Twitter to respond to fans who were feeling less than happy about the April 7 announcement, and he kept it a buck about what went down.”